1. Field of the Invention
This invention relates to hydrocarbon conversion. It more particularly relates to an improved process for upgrading the octane number of a gasoline-boiling fraction.
2. Description of the Prior Art
Modern petroleum refinery technology is capable of reforming naphthas which are paraffinic and/or naphthenic in nature in order to increase the octane number thereof. Such reforming is traditionally carried out in contact with platinum type reforming catalysts and is a widely used commercial refinery process. Platinum reforming is particularly well suited for use in upgrading naphthenic fractions to aromatic fractions.
More recently it has been discovered that naphtha fractions, which are not particularly naphthenic or which may contain substantially no naphthenes at all, can be conveniently aromatized in good, commercially acceptable yields by converting such feeds, under relatively severe conditions, in contact with ZSM-5 and related crystalline aluminosilicate zeolite catalysts. Highly aromatic liquid yields of upwards of 30 percent have been readily achieved by this process.
In a similar process, aromatic containing feeds, such as reformates, have been upgraded in a manner whereby their aromatic contents have been increased by contacting such feeds with this same kind of catalyst, that is, ZSM-5 and related intermediate pore aluminosilicate zeolites. This latter process is believed to operate by selectively cracking aliphatics in the feed to produce active fragments at least some of which alkylate existing aromatics in the feed whereby increasing the highly desirable, high octane aromatic content thereof while decreasing the less desirable, low octane paraffin content thereof.
The principal differences between these two processes are the feeds being converted and the severity of conversion conditions. The first aforesaid process is principally valuable for converting a predominantly aliphatic feed and operates at about 650.degree. to 1500.degree. F. at a space velocity of about 1 to 15 WHSV. The second aforesaid process is principally valuable for converting a feed which is already rich in aromatics and operates at about 500.degree. to 1000.degree. F. It will be seen that the operating conditions overlap to some extent as do the feeds. It is probable that some cracking-alkylation and some aromatization take place in both processes. The distinction between the processes is perhaps better expressed as one of conversion predominance with the more severe conditions favoring new aromatic ring formation and the less severe conditions favoring alkylation of preformed or newly created aromatic rings.
In either case, it has been shown that the processes referred to above are improved when the ZSM-5 catalyst is modified to include a certain proportion, e.g. up to about 10 weight percent, of zinc or cadmium, or other similarly promoting metal therein. Such metal is suitably incorporated with the zeolite by cation exchange, impregnation and/or vapor deposition. It has also been shown that the further inclusion of copper into such a catalyst composition is beneficial especially in that the loss during regeneration of zinc and/or cadmium is significantly reduced thereby.
The following patents comprise at least a partial list of those patents directed to processes such as described above for upgrading hydrocarbon fractions over acidic crystalline aluminosilicate zeolites to improved gasoline products.
U.S. Pat. No. 3,756,942 discloses increasing the aromatic content of a light gasoline formed by fluid catalytic cracking (FCC) by conversion of the gasoline over ZSM-5 zeolite.
U.S. Pat. No. 3,760,024 also discloses a process for producing aromatic compounds by contacting C.sub.2 -C.sub.4 paraffins, olefins or mixtures thereof with ZSM-5 and recovering the aromatic compounds.
U.S. Pat. No. 3,775,501 discloses improving the yield of aromatics from a hydrocarbon feed selected from the group consisting of aliphatic olefins and paraffins by contacting the hydrocarbon feed in air or oxygen with a crystalline aluminosilicate zeolite such as ZSM-5.
U.S. Pat. No. 3,827,968 discloses a process for manufacturing gasoline by contacting C.sub.2 -C.sub.5 olefins with ZSM-5 under such conditions as to oligomerize the olefins and subsequently passing the oligomerized olefins over ZSM-5 at aromatizing conditions to form a product having an enhanced aromatic content.
U.S. Pat. No. 3,890,218 discloses a process for upgrading the octane number of hydrocarbon fractions boiling in the naphtha range and having a low octane number by contacting the naphtha fraction over an intermediate pore zeolite such as ZSM-5 in which the activity of the zeolite has been modified such as by steaming so as to increase the high octane liquid yield by shape selective cracking-alkylation mechanism and an aliphatic hydrocarbon aromatization process. The process is preferably operated at conditions which are intermediate between the optimum conditions for the respective conversion mechanisms. Among the feeds which are useful in the aforementioned patent are cracked gasolines. The preferred feeds are hydrocarbon compositions containing 0 to 20 wt.% aromatics, predominantly C.sub.5 -C.sub.8 aromatics, and about 60-100 wt.% straight and branched chain paraffins and olefins with minimal amounts of naphthenes.
U.S. Pat. No. 3,953,366 discloses a process for the aromatization of hydrocarbons and the alkylation of aromatic rings by contacting a hydrocarbon feed such as a cracked gasoline fraction with ZSM-5 and related zeolites which has rhenium deposited thereon.
U.S. Pat. No. 3,960,978 discloses converting gaseous C.sub.2 -C.sub.5 olefins to an olefinic gasoline by passing the olefin feed over a ZSM-5 zeolite catalyst. The zeolite catalyst can be steamed to a low alpha activity value.
U.S. Pat. No. 4,021,502 discloses producing a gasoline by passing a feed stock of C.sub.2 -C.sub.5 olefins or mixtures thereof with C.sub.1 -C.sub.5 paraffins over ZSM-4, ZSM-12, ZSM-18, chabazite or zeolite beta.
U.S. Pat. No. 4,227,992 discloses a process for separating ethylene from light olefins by contact with ZSM-5 under conditions such that the C.sub.3 + olefins are converted to both gasoline and fuel oil.
U.S. Pat. No. 4,396,497 describes the treatment of gasoline boiling range hydrocarbons to increase the octane number thereof by contact with a gamma alumina catalyst.
Another process recently used to increase the octane of gasoline boiling fractions involves the addition of ZSM-5 and related intermediate pore zeolites to the conventional cracking catalyst such as zeolites of the X or Y faujasite variety during the cracking of gas oils to gasoline products. Examples of patents which describe such a process include U.S. Pat. Nos. 3,894,931; 3,894,903; and 3,894,934.
One important consideration involved in the upgrading of gasoline fractions in addition to boosting the octane thereof is obtaining the highest possible liquid yield. Thus, although the technology referred to above is excellent in upgrading the quality of gasoline boiling range fragments, hydrocarbon conversion over ZSM-5 under cracking, alkylation or aromatizing conditions has resulted in substantial loss of gasoline yield in the form of light gases, i.e., C.sub.1 -C.sub.4.